Performance Of Hot-Dip Galvanized Steel Products
Performance ofHot-Dip Galvanized Steel ProductsIn the Atmosphere, Soil, Water, Concrete, and More
Table of ContentsIntroductionSteel Corrosion and Corrosion ProtectionThe Corrosion ProcessGalvanic CorrosionCorrosion of SteelHow Zinc Protects Steel from CorrosionThe Hot-Dip Galvanizing ProcessSurface PreparationGalvanizingInspectionPhysical Properties of Hot-Dip Galvanized SteelThe Metallurgical BondImpact and Abrasion ResistanceComplete, Uniform CoveragePerformance of Galvanized SteelIn the AtmosphereIn SoilsIn Fresh WaterIn Sea Water and Salt Spray ExposureIn Chemical SolutionsIn Contact With Treated WoodIn ConcreteIn Extreme TemperaturesIn Contact With Other MetalsSummary1111223334444555788899101112Copyright 2010 American Galvanizers Association. The material provided herein has been developed to provide accurateand authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information onlyand is not intended as a substitute for competent professional examination and verification as to suitability and applicability.The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making useof this information assumes all liability arising from such use.
IntroductionCorrosion and repair of corrosion damage are multi-billion dollar problems. Estimates show metallic corrosion coststhe United States approximately 297 billion annually, or about 3% of the national GDP. Although corrosion is a naturalphenomenon, and can never be completely eliminated, utilizing adequate corrosion protection systems in harshenvironments can drastically reduce the costs. Hot-dip galvanizing after fabrication is a cost effective, maintenancefree corrosion protection system that lasts for decades even in the harshest environments. For more than 100 years,hot-dip galvanized steel has been utilized extensively to combat corrosion in major industrial environments includingpetro-chemical, transportation, and public utilities.The zinc of the hot-dip galvanized coating is more corrosion resistant than bare iron and steel. Similar to steel, zinccorrodes when exposed to the atmosphere; however, zinc corrodes at a rate approximately 1/30 of that for steel.Also like steel, zinc corrodes at different rates depending on its environment. Therefore, the performance of hot-dipgalvanized steel varies from environment to environment. Environments in which galvanized steel is commonly usedinclude indoor and outdoor atmospheres, the storage of hundreds of different chemicals, in fresh water, sea water, soils,concrete, and/or in conjunction with other metals, treated wood, or extreme temperatures. Because of the many yearsgalvanizing has been used for corrosion protection, a wealth of real-world, long-term exposure data on zinc coatingperformance in a wide variety of environments is available. Because hot-dip galvanized steel is used in so manydifferent applications, it is important to understand what factors affect its performance in each of these environments.Steel Corrosion and ProtectionThe Corrosion ProcessConventional CurrentElectronsElements are rarely found in a pure metal state. Rather,they are found in chemical combinations with one or morenonmetallic elements. These chemical combinations arecommonly known as ore.Significant energy must be expended to reduce the ore topure metal. This energy can be applied via metallurgicalor chemical means. Additional energy also may be usedin the form of cold-working or heating and casting totransform the pure metal into a working shape. Corrosioncan be viewed simplistically as the tendency for a metalto revert to its natural, lower energy state, ore. From athermodynamic perspective, the tendency to decrease inenergy is the main driving force behind metallic corrosion.ExternalCircuit CathodeElectrons-AnodeElectrolyteFigure 1: Bi-metallic coupleGalvanic CorrosionAnode- The electrode wheregalvanic reaction(s)generate electrons.Corrosion occurs at theanode.There are two primary types of galvanic cells that causecorrosion: the bi-metallic couple and the concentrationcell. A bi-metallic couple (Figure 1) is like a battery,consisting of two dissimilar metals immersed in anelectrolyte solution. An electric current (flow of electrons)is generated when the two electrodes are connected byan external, conductive path.A concentration cell consists of an anode and cathodeof the same metal or alloy and a return current path.The electromotive force is provided by a difference inconcentration of the surfaces through the external path.There are four elements necessary for corrosion to occurin a galvanic cell:Cathode- The electrode that receiveselectrons. The cathode isprotected from corrosion.Electrolyte- This is the conductor throughwhich current is carried.Electrolytes include aqueoussolutions or other liquids.Return Current Path- This is the metallic pathwayconnecting the anode tothe cathode. It is often theunderlying metal.American Galvanizers Association1
All four elements, anode, cathode, electrolyte and returncurrent path, are necessary for corrosion to occur.Removing any one of these elements will stop the currentflow and galvanic corrosion will not occur. Substituting adifferent metal for the anode or cathode may cause thedirection of the current to change, resulting in a switch asto the electrodes experiencing corrosion.The Galvanic Series of Metals (Figure 2) lists metals andalloys in decreasing order of electrical activity. Metalsnearer the top of the table often are referred to as lessnoble metals and have a greater tendency to lose electronsthan the more noble metals found lower on the list.CORRODED ENDAnodic or less adTinNickelBrassBronzesCopperStainless Steel (passive)SilverGoldPlatinumPROTECTED ENDCathodic or more noble(ELECTROPOSITIVE)Cathodic protection canoccur when two metals areelectrically connected. Anyone of these metals oralloys will theoreticallycorrode while offeringprotection to any other thatis lower in the series, solong as both are electricallyconnected.However, in actualpractice, zinc is by far themost effective in thisrespect.CAAAACCMosaic of anodes and cathodes,electrically connected by theunderlying steel.ACCCACAAAMoisture in the air provides theelectrical path between anodesand cathodes. Due to differencesin potential, electric current beginsto flow as the anodic areas areconsumed. Iron ions produced atthe anode combine with theenvironment to form the flaky ironoxide known as rust.CCAs anodic areas corrode, newmaterial of different compositionand structure is exposed. Thisresults in a change of electricalpotentials and changes thelocation of anodic and cathodicsites. Over time, previouslyuncorroded areas are attackedand uniform surface corrosionresults. This continues until thesteel is entirely consumed.Figure 3: Changes in cathodic and anodic areasThe rate at which metals corrode is controlled byfactors such as the electrical potential and resistancebetween anodic and cathodic areas, pH of the electrolyte,temperature, and humidity.The corrosion products of steel are oxide particles and havea distinctive brown/red color: rust. Just a small amountof these particles can cause an uncoated steel surface toappear corroded. Steel corrodes naturally when exposedto the atmosphere, but the corrosion process accelerateswhen concentration cells are active on the surface.How Zinc Protects Steel from CorrosionThe reason for the extensive use of hot-dip galvanizingis the two-fold protective nature of the coating. As abarrier coating, it provides a tough, metallurgicallybonded zinc coating that completely covers the steelsurface and seals the steel from the corrosive action ofthe environment. Additionally, zinc’s sacrificial behaviorprotects the steel, even where damage or a minordiscontinuity in the coating occurs.Figure 2: Galvanic Series of MetalsBarrier ProtectionCorrosion of SteelBarrier protection is perhaps the oldest and most widelyused method of corrosion protection. It acts by isolatingthe base metal from the environment. Two importantproperties of barrier protection are adhesion to the basemetal and abrasion resistance. Paint is one example ofa common barrier protection system.The actual corrosion process that takes place on a piece ofuncoated steel is very complex. Factors such as variationsin the composition/structure of the steel, presence ofimpurities due to the higher instance of recycled steel,uneven internal stress, and/or exposure to a non-uniformenvironment all affect the corrosion process.It is very easy for microscopic areas of the exposed steel tobecome relatively anodic or cathodic to one another. A largenumber of such areas can develop in a small section of theexposed steel. Further, it is highly possible several differenttypes of galvanic corrosion cells are present in the samesmall area of an actively corroding piece of steel.As the corrosion process progresses, corrosion productsmight tend to build up in certain areas of the metal. Thesecorrosion products have different elemental compositionsthan their original state. The new compositions exposedon the surface lead to changes in the anodic and cathodicareas. As the change in anodic and cathodic areas occur,previously uncorroded areas of the metal can be attackedand corrode. This eventually will result in overall corrosionof the steel surface (Figure 3).2Zinc PatinaThe barrier and cathodic protection prevent corrosionof the steel itself. The zinc metal is protected by theformation of a patina layer on the surface of the coating.The zinc patina is formed by the conversion of zincmetal into corrosion products through interaction withthe environment. The first products formed includezinc oxide and zinc hydroxide. Later in the corrosioncycle these products interact with carbon dioxide in theenvironment to form zinc carbonate. The zinc carbonateis a passive, stable film that adheres to the zinc surfaceand is not water soluble so it does not wash off in the rainor snow. This zinc carbonate layer corrodes very slowlyand protects the zinc metal underneath. The formationof the zinc carbonate turns the zinc coating to a dullgray color. The long term corrosion protection of the zinccoating depends on the formation of the patina layer.American Galvanizers Association
Cathodic ProtectionCathodic protection is an equally important methodfor preventing corrosion. Cathodic protection requireschanging an element of the corrosion circuit byintroducing a new corrosion element, thus ensuringthe base metal becomes the cathodic element of thecircuit.There are two major variations of the cathodic methodof corrosion protection. The first is the sacrificial anodemethod. In this method a metal or alloy anodic tothe base metal to be protected is placed in the circuitand becomes the anode. The protected base metalbecomes the cathode and does not corrode. Theanode corrodes, thereby providing the desired sacrificialprotection. Zinc is anodic to iron and steel; thus,the galvanized coating provides cathodic corrosionprotection as well as barrier protection.The other form of cathodic protection is called theimpressed current method. In this method, an externalcurrent source is used to impress a cathodic charge onall the iron or steel to be protected. While such systemsgenerally do not use a great deal of electricity, they oftenare very expensive to install and maintain.Pickling - A dilute solution of hot sulfuric acid or ambienttemperature hydrochloric acid removes mill scaleand iron oxides (rust) from the steel surface. As analternative to or in conjunction with pickling, this stepcan also be accomplished using abrasive cleaning, airblasting sand, metallic shot, or grit onto the steel.Fluxing - The final surface preparation step in thegalvanizing process serves two purposes. It removesany remaining oxides and deposits a protective layeronto the steel to prevent any further oxides from formingon the surface prior to galvanizing.Flux is applied in two different ways; wet or dry. In thedry galvanizing process, the steel or iron is dipped orpre-fluxed in an aqueous solution of zinc ammoniumchloride. The material is then dried prior to immersionin molten zinc. In the wet galvanizing process, a layer ofliquid zinc ammonium chloride is floated on top of themolten zinc. The iron or steel being galvanized passesthrough the flux on its way into the molten zinc (Figure 4).Dry utionDryingZincbathCooling andinspectionWet galvanizingFluxThe Hot-Dip Galvanizing ProcessThe hot-dip galvanizing process consists of three basicsteps: surface preparation, galvanizing, and inspection.Surface PreparationDegreasingRinsingPicklingRinsingZinc bathCooling andinspectionFigure 4: Hot-dip galvanizing processSurface preparation is the most important step in theapplication of any coating. In most instances, incorrect orinadequate surface preparation is the cause of a coatingfailure before the end of its expected service lifetime.The surface preparation step in the galvanizing processhas its own built-in means of quality control because zincwill not metallurgically react with an unclean steel surface.Any failures or inadequacies in surface preparation willimmediately be apparent when the steel is withdrawn fromthe molten zinc, because the unclean areas will remainuncoated and immediate corrective action must be taken.Surface preparation for galvanizing consists of three steps:degreasing, acid pickling, and fluxing.Degreasing - A hot alkali solution, mild acidic bath, orbiological cleaning bath removes organic contaminantssuch as dirt, paint markings, grease, and oil fromthe steel surface. Degreasing baths cannot removeepoxies, vinyls, asphalt, or welding slag; thus,these materials must be removed by grit-blasting,sand-blasting, or other mechanical means before thesteel is sent to the galvanizer.GalvanizingIn the true galvanizing step of the process, the materialis completely immersed in a bath of molten zinc. Thebath contains at least 98% pure zinc and is heated toapproximately 840 F (449 C). Zinc chemistry is specifiedby ASTM B 6.While immersed in the kettle, the zinc reacts with theiron in the steel to form a series of zinc/iron intermetallicalloy layers. Once the fabricated items coating growthis complete, they are withdrawn slowly from thegalvanizing bath, and the excess zinc is removed bydraining, vibrating, and/or centrifuging.The metallurgical reaction will continue after thearticles are withdrawn from the bath, as long as thearticle remains near bath temperature. Articles arecooled either by immersion in a passivation solution orwater or by being left in open air.Hot-dip galvanizing is a factory-controlled processperformed under any climate conditions. Most brushand spray-applied coatings depend upon properclimate conditions for correct application. DependenceAmerican Galvanizers Association3
on atmospheric conditions often translates into costlyconstruction delays. The galvanizer’s ability to work in anyclimate conditions provides a higher degree of assuranceof on-time delivery; furthermore, no climate restrictionsmeans galvanizing can be completed quickly and withshort lead times.The inspection process for galvanized items also requiresminimal labor. This is important because the inspectionprocess required to assure the quality of many brush- andspray-applied coatings is highly labor-intensive and requiresexpensive skilled labor.Once a job has been delivered and accepted at the galvanizer’splant, there is one point of responsibility for ensuring thematerial leaves the plant properly galvanized. That point ofresponsibility is the galvanizer.Physical Properties of Hot-DipGalvanized SteelThe Metallurgical BondWithdrawal of a steel article from the zinc bath.InspectionThe inspection of hot-dip galvanized steel is simple andfast. The two properties of the coating closely scrutinizedare coating appearance and coating thickness. A variety ofsimple physical and laboratory tests may be performed todetermine thickness, uniformity, adherence, and appearance.Galvanizing forms a metallurgical bond between the zincand the underlying steel or iron, creating a barrier that ispart of the metal itself. During galvanizing, the molten zincreacts with the iron in the steel to form a series of zinciron alloy layers. Figure 5 is a photomicrograph of a typicalgalvanized coating microstructure consisting of three alloylayers and a layer of pure metallic zinc.The galvanized coating is tightly bonded to the underlyingsteel, at approximately 3,600 pounds per square inch(psi). Other coatings typically offer bond strengths of 300600 psi, at best.Products are galvanized according to long-established,accepted, and approved standards of ASTM, the CanadianStandards Association (CSA), the International Organizationfor Standardization (ISO), and the American Association ofState Highway and Transportation Officials (AASHTO). Thesestandards cover everything from minimum required coatingthicknesses for various categories of galvanized items to thecomposition of the zinc metal used in the process.Figure 5: Photomicrograph of galvanized coatingImpact and Abrasion ResistanceThe coating microstructure displayed in Figure 5 alsoindicates the hardness of each layer, expressed by aDiamond Pyramid Number (DPN). The DPN is a progressivemeasure of hardness, which means the higher the numberthe greater the hardness. Typically, the Gamma, Delta,and Zeta layers are harder than the underlying steel.The hardness of these inner layers provides exceptionalprotection against coating damage through abrasion.The Eta layer of the galvanized coating is quite ductile,providing the coating with some impact resistance.This sculpture was hot-dip galvanized to keep it beautiful in acorrosive outdoor environment.4American Galvanizers Association
Hardness, ductility, and bond strength combine to providethe galvanized coating with unmatched protection againstdamage caused by rough handling during transportationto and/or at the job site as well as during its service life.Furthermore, because galvanizing provides more thanjust barrier protection, even if the impermeable coating isphysically damaged, it will continue to provide cathodicprotection to the exposed steel. Exposed areas of steelup to ¼” in size will be protected from corrosion by thesurrounding zinc until all of the coating is gone (Figure 6).GALVANIZED STEELThis is what happens to ascratch on galvanized steel.The zinc coating sacrificesitself slowly by galvanic actionto protect the base steel.This sacrificial action continuesas long as any zinc remainsin the immediate area.Figure 8: Full Corner ProtectionCoating damage is most likely to occur at edges andoften corrosion beings in the interior of hollow structures,so these areas are where added protection is needed.Figure 6: Zinc protects scratched base steelThe toughness of the galvanized coating is extremelyimportant, since barrier protection is dependent uponcoating integrity. Other coatings damage easily duringshipment or through rough handling on the job site.Furthermore, all organic forms of barrier protection,such as paint, are permeable to some degree (pinholes),which means electrolytes in the environment will beginto damage even an intact coating. Figure 7 shows howcorrosion will begin and immediately progress at ascratch or gap in a paint coating.PAINTED STEELThis is what happens to ascratch on painted steel. Theexposed steel corrodes andforms a pocket of rust. Becauserust is much more voluminousthan steel, the pocket swells andlifts the paint film from themetal surface to form a blister.Both the corrosion pit and theblister continue to grow.Figure 7: Underfilm corrosion causes paint to peel and flakeComplete, Uniform CoverageThe metallurgical reaction that occurs between zinc andsteel is a diffusion process, whi
The Hot-Dip Galvanizing Process The hot-dip galvanizing process consists of three basic steps: surface preparation, galvanizing, and inspection. Surface Preparation Surface preparation is the most important step in the application of any coating. In most instances, incorrect or inadequate surface preparation is the cause of a coating
This specification covers the painting of hot dip galvanized steel other than sheet and wire. The requirements for the painting of hot dip galvanized sheet are contained in ISO 12944 Parts 4 and 5. 7.1 All hot dip galvanized steel to be painted shall be certified as conforming to the required hot dip galvanizing quality standard, prior to painting.
the Inspection of Hot-Dip Galvanized Products Seminar. SUMMARY This paper will address technical issues with the use of hot-dip galvanized steel in bridge construction. Some key technical issues that will be addressed are the slip factor of hot-dip galvanized, painted, and metallized coatings in slip .
1. AASHTO M111, Zinc (Hot Dip Galvanized) Coatings on Iron and Steel Products 2. AASHTO M232, Zinc Coating (Hot Dip) on Iron and Steel Hardware c. American Galvanizers Association (AGA) 1. The Inspection of Products Hot Dip Galvanized After Fabrication 2. Powder Coating over Hot Dip Galvanized Steel, Powder Coating Journal, Feb 2004,
Hot-Dip Galvanizing for Corrosion Protection: A Guide to Specifying and Inspecting Hot-Dip Galvanized ReinforcingSteel Table of Contents: 2004 American Galvanizers Association. The material in this publication has been developed to provide accurate and authoritative information about painting over hot-dip galvanized steel after fabrication.
publication Inspection of Products Hot-Dip Galvanized After Fabrication which may be obtained from your galvanizer or the AGA. Iron and steel articles hot-dip galvanized after fabrication may range in size from small pieces of hardware, such as bolts and washers, to large, welded steel assemblies, weighing several tons.
ance of hot dip galvanized structural steel in an obscured ap-plication is different from that where a product is used as an ar-chitectural feature. An awareness of the end use of the product and the capability of the hot dip galva-nizing process is essential for good inspection. Inspection of hot dip galvanized products, as the final step in the
Hot-dip galvanized coil: it has great resistance against atmospheric corrosion. Its dimensions are defined according to the customer request. This steel is much used by automobile manufacturers. 9 ONLINE INSPECTION In this stage, the inspection and checking of the requests made by the customer are performed. 1 FULL HARD COILS Hot-dip .
these design practices along with those listed in ASTM A385 Practice for Providing High Quality Zinc Coatings (Hot-Dip) , will not only produce optimum quality galvanized coatings, but also help reduce costs and improve turnaround times. One key to providing the best design for the hot-dip galvanizing
